Allright gentleman, going to split my comments into two posts to fit in the coffee breaks. First up:
I would say go ahead an start using the more accurate modifiers when you have a chance, we start see how they feel for balance / play purposes. The goal of these spells is to affect tactical battles to be more geographically interesting. We are also trying to keep with the larger game spell idea of having each one be uniquely useful. Given the current limitations of the engine and tags, I suggest the following general ideas:
Grasping Hands: short range, stops units, deals damage, shorter duration
Freezing Rain: medium range, slows units and decreases combat effectiveness (accuracy, combat speed, dodge, etc), shorter duration
Wall of Flame: medium range, deals damage, explosion moves unit back 2 tiles, blocks tile for brief duration
Howling Wind: unlimited range, decreases combat effectiveness [accuracy & damage], preferably by giving Dodge bonus to targets (this makes is specially for countering ranged units), longer duration, unit enchantment so sticks to target when they move
I know the air spell is not geographic, but I think that exception helps keep everything tactically interesting.